Optical microcavities confine light to small volumes by resonant recirculation and have demonstrated potential use as microscopic light emitters, lasers, and sensors (K. J. Vahala, Nature Vol. 424, pp. 839-846, 2004). The recirculation imposes geometry-dependent boundary conditions on wavelength and propagation direction of the light kept inside the microcavity. Accordingly, only certain optical modes, the so-called “cavity modes”, can be populated. Since the energy levels of these allowed modes depend crucially on geometry and optical properties of the microcavities, the latter comprise very sensitive microscopic optical sensors that can be used for example to sense forces (e.g. by deformation of the cavity) or changes in chemical concentration (e.g. by a corresponding change of the refractive index in close vicinity of the microcavity). Similarly, microcavities can be used for biomolecular detection, e.g. by surface adsorption of specifically binding molecules to or into a microcavity and the resultant change of the refractive index around or inside of the cavity.
The confinement of light inside of a microcavity requires a highly reflective boundary between the microcavity and surrounding. This can be achieved for example via total internal reflection, similarly to the guidance of light inside of an optical waveguide. As shown in FIG. 1, total internal reflection can occur if the refractive index of the microcavity, ncav, is larger than that of its surrounding, nenv, i.e. ncav>nenv. However, even in this case, total internal reflection occurs only for angles α above a so-called “critical angle” αcrit=arcsin (nenv/ncav), where α is measured from the local surface normal inside of the cavity, where the reflection occurs. Such simple considerations remain valid as long as surface roughness is negligible as compared to the wavelength of the light trapped inside the microcavity. Accordingly, one general lower size limit of microcavities is given by the precision to which smooth surfaces can be prepared.
Another obstacle for utilization of microcavities is directly related to the requirement of a highly reflective interface between microcavity and surrounding. Since the path of light is reversible in absorption-free media, the interface will be also highly reflective for those light beams 20 that impinge onto the interface from the surrounding. Accordingly, just those optical modes inside the cavity, which fulfill the requirement of high reflectivity and thus provide long light storage potential, cannot be easily populated by light accessing the microcavity from the outside.
Vollmer and coworkers (F. Vollmer et al., Applied Physics Letters Vol. 80, pp. 4057-4059, 2002) used evanescent field coupling between the uncoated core of an optical fiber and a silica microsphere for population of the cavity modes inside of the microsphere. In this case, photons can transit from the high refractive index core of the fiber to the high refractive index interior of the microsphere via tunnelling through a small air gap. The air gap should be in the range of a few nanometers and has to be precisely controlled. For this reason, the microsphere has to be fixed to a solid mount, which in the case of Vollmer et al. also was used as microfluidic device for biosensing application. Vollmer et al. were able to demonstrate cavity mode biosensing via adsorption of bovine serum albumine (BSA) onto the outer surface of silica spheres with diameters of 300 μm. They showed that the sensitivity of their sensor scales with 1/R, where R is the particle radius.
Kuwata-Gonokami and coworkers (M. Kuwata-Gonokami et al., Japanese Journal of Applied Physics Vol. 31, pp. L99-L101, 1992) used dye-doped polystyrene (PS) microspheres for populating cavity modes. The dye-containing microspheres were radiated with ultrashort laser pulses to excite the dye molecules. The pump laser pulse was incident onto the microsphere surface at a small incidence angle α, so that the light could penetrate into the optically denser microsphere with small loss only (˜5-10%). The excited dye molecules inside of the microcavity re-radiate fluorescent light into arbitrary directions, i.e. also into those which fulfill the condition of total internal reflection. Accordingly, all cavity modes which fall into the emission wavelength range of the dye molecules became populated. At high pump intensities microcavity lasing was observed.
Halas and coworkers have suggested core-shell particles of much smaller size consisting of a non-metallic core and a metallic shell for optical biosensing (West et al., U.S. Pat. No. 6,699,724 B1). They studied in particular the size regime from few tens to several hundreds of nanometers, i.e. particles with an outer diameter of <1 μm. The conductive shell of such particles can be optically excited at the so-called “plasma frequency”, which corresponds to a collective oscillation of the free electrons of the shell. While the plasma frequency of solid metal particles shows only marginal dependence on the particle size and is basically given by the physical properties of the bulk material, such as electron density and effective electron mass, Halas et al. were able to demonstrate that in the case of core-shell particles the position of the plasma frequency can be tuned over a wide range from the visible to the near infrared solely by changing the ratio between core and shell radii of the particles (N. Halas, Optics & Photonics News, Vol. 13, Iss. 8, pp. 26-31, 2002; S. J. Oldenburg et al., Chemical Physics Letters, Vol. 288, pp. 243-247, 1998). Halas et al. suggested to use such particles as biosensors by tuning the plasma frequency into a frequency range where it could support surface enhanced Raman emission of organic molecules adsorbed on the outer shell surface. The Raman emission then can serve as qualitative measure of protein adsorption. It must be noted, that Halas et al. use the core-shell character of the fabricated particles solely for tuning of the plasma frequency but not for generation or utilization of microcavity modes. In the course, they do not suggest to embed any kind of fluorescent material into the non-metallic particle cores for population of such modes.
Besides closed microcavities, also the utilization of open microcavities has been suggested for biosensing. These microcavities comprise microscopic vacancies in a thin metallic film. The light is confined only in the plane of the thin film, but free in perpendicular direction. Blair and coworkers (Y. Liu et al., Nanotechnology Vol. 15, pp. 1368-1374, 2004; Y. Liu & S. Blair, Proceedings of SPIE Vol. 5703, pp. 99-106, 2005) studied fluorescent enhancement of dye-labeled proteins adsorbed into nanocavities patterned in a thin gold film. They observed fluorescent enhancement by a factor of 2 and an increase in quantum yield by a factor of 6. In contrast to what will be described below, there is no additional active medium, such as dye molecules or quantum dots for population of the cavity modes except those fluorophore labels attached to the analyte. Accordingly, the observed effects are rather weak and observable only with large scale assemblies of individual microcavities.
Scherrer and coworkers (O. Painter et al., Science Vol. 284, pp. 1819-1821, 1999) achieved the so far smallest microcavity volumes of 0.03 cubic micrometers with a single defect in a two-dimensional photonic crystal and confined light with a wavelength of 1.55 μm to it. In this calculation of the cavity volume, however, they did not include the overall size of the periodic structure that is required to keep the photons trapped. The latter will probably hamper the down sizing of sensors based on photonic crystals into the sub-micron regime.
There exist a variety of other methods for label-free biosensing based on plasma excitations of metal particles or thin metal films. In these cases, an incoming light wave is used to launch a free propagating or localized surface plasmon (which corresponds to a collective oscillation of the free electrons of the metal). The plasmon in turn produces an evanescent electromagnetic wave in the close environment of the metal film or metal particle. When the dielectric properties in this environment are altered, e.g. due to biomolecular adsorption, the plasmon oscillation alters its resonance position. Accordingly, this shift can be used as read-out signal of a label-free optical biosensor.
Examples of approaches utilizing localized plasmon effects are given in US 2003/0174384 A1, EP 0 965 835 A2 and Sensors and Actuators B 2000, 63, pp. 24-30. An example for utilization of free-travelling plasmons is given by the BIAcore system from Pharmacia Biosensor, Piscataway, N.J., USA. In none of these cases, however, microcavity modes have been suggested for amplification of said plasmon effects.